Soluble Fas (sFas) has the ability to block Fas-mediated apoptosis, suggesting that sFas at tumor sites might inhibit tumor cell-killing by immune effector cells. We examined the sFas level in pleural effusion associated with lung cancer. The level of sFas in malignant pleural effusion was significantly higher than those in transudate and tuberculous pleural effusion. There was no significant difference in the sFas concentration among various histological types of lung cancer. The cytotoxicity mediated by anti-Fas agonistic antibody against Jurkat cells was inhibited by the addition of malignant pleural effusion, being inversely correlated with the sFas concentration. When Fas expression was examined using flow cytometry, eight of ten (80%) lung cancer cell lines expressed cell surface Fas. On the other hand, sFas protein and mRNA were detected in six of ten (60%) lung cancer cell lines, but there was no correlation between Fas and sFas expression. Furthermore, although the expressions of Fas and sFas were clearly detected in tumor cells derived from malignant effusion, the sFas expression was down-regulated in an in vitro culture. These results suggest that sFas in malignant pleural effusion is at least in part produced by lung cancer cells, and might play a role in local immunosuppression by tumor cells.